Processes for preparing crosslinked resins and toner compositions therefrom

ABSTRACT

A reactive melt mixing process for preparing toner resin, comprising: melting a reactive base resin, thereby forming a polymer melt; adding to said polymer melt a free radical initiator compound and optionally a vinyl monomer compound to form a second melt; and heating and mixing under high shear said second melt with optional addition of a vinyl monomer compound to form a partially crosslinked and partially gelled toner resin.

CROSS REFERENCE TO COPENDING APPLICATIONS AND ISSUED PATENTS

Attention is directed to commonly owned and assigned copendingapplications: U.S. Ser. No. 07/814,641 (D/9 1117) filed Dec. 30, 1991,entitled "Reactive Melt Mixing Process for Preparing Crosslinked TonerResin"; U.S. Ser. No. 08/035,609 (D/91117D) filed Mar. 23, 1993,entitled "Reactive Melt Mixing Process For Preparing Crosslinked TonerResin"; U.S. Ser. No. 08/035,398 (D/91117QD) filed Mar. 23, 1993,entitled "Crosslinked Toner Resins Formed by Reactive Melt Mixing"; U.S.Ser. No. 08/159,176 (D/92555) filed Nov. 30, 1993, entitled "UnsaturatedPolyesters"; and U.S. Ser. No. 08/131,250 (D/92556) filed Oct. 4, 1993,entitled "Melt Mixing Processes".

Attention is directed to commonly owned and assigned U.S. Pat. No.5,227,460, U.S. Ser. No. 07/814,782 (D/91117Q) filed Dec. 30, 1991,entitled "Crosslinked Toner Resins".

BACKGROUND OF THE INVENTION

The present invention is generally directed to processes for thepreparation of toner resins and toners. More specifically, the presentinvention relates to melt mixing processes, batch or continuous, butpreferably continuous processes such as, for example, reactive extrusionfor preparing crosslinked toner resins. The present invention alsorelates to processes for crosslinking reactive linear resins for thepreparation of crosslinked toner resins that can be selected forapplication in heat fixable toners with superior fusing and vinyl offsetperformance.

Toner utilized in development in the electrographic process is generallyprepared by mixing and dispersing a colorant and a charge enhancingadditive into a thermoplastic binder resin, followed bymicropulverization. As the thermoplastic binder resin, numerous polymersare known, including polystyrenes, styrene-acrylic resins,styrene-methacrylic resins, polyesters, epoxy resins, acrylics,urethanes and copolymers thereof. As the colorant, carbon black,magnetite and various colored pigments may be selected, and as thecharge enhancing additive, alkyl pyridinium halides, distearyl dimethylammonium methyl sulfate, metallic alkyl salicylates, and the like areknown.

To fix the toner to a support medium, such as a sheet of paper ortransparency, hot roll fixing is commonly used. In this method, thesupport medium carrying a toner image is transported between a heatedfuser roll and a pressure roll, with the image face contacting the fuserroll. Upon contact with the heated fuser roll, the toner melts andadheres to the support medium, forming a fixed image. Such a fixingsystem is very advantageous in heat transfer efficiency and isespecially suited for high speed electrophotographic processes.

Fixing performance of the toner can be characterized as a function oftemperature. The lowest temperature at which the toner adheres to thesupport medium is called the Cold Offset Temperature (COT), and themaximum temperature at which the toner does not adhere to the fuser rollis called the Hot Offset Temperature (HOT). When the fuser temperatureexceeds HOT, some of the molten toner adheres to the fuser roll duringfixing and is transferred to subsequent substrates containing developedimages, resulting for example in blurred or extraneous images. Thisundesirable phenomenon is called offsetting. Between the COT and HOT ofthe toner, is the Minimum Fix Temperature (MFT) which is the minimumtemperature at which acceptable adhesion of the toner to the supportmedium occurs, as determined by, for example, a creasing test. Thedifference between MFT and HOT is called the Fusing Latitude.

The hot roll fixing system and a number of toners used therein, however,exhibit several problems. First, the binder resins in the toners canrequire a relatively high temperature in order to be affixed to thesupport medium. This may result in high power consumption, low fixingspeeds, and reduced life of the fuser roll and fuser roll bearings.Second, offsetting phenomena can be a problem. Third, toner containingvinyl type binder resins such as styrene-acrylic resins may have anadditional problem known as vinyl offset. Vinyl offset occurs when asheet of paper or transparency with a fixed toner image comes in contactfor a period of time with, for example, a polyvinyl chloride (PVC)surface containing a plasticizer used in making the vinyl materialflexible such as, for example, in vinyl notebook binder covers, and thefixed image adheres to the PVC surface.

Thus, there remains a need for toner resins with low fix temperature andhigh offset temperature and broad fusing latitude, superior ornonexistent vinyl offset property, and efficient and economic processesfor the preparation of such resins.

In order to prepare lower fix temperature resins for toner, themolecular weight of the resin may be lowered. Low molecular weight andamorphous polyester resins and epoxy resins have been used to preparelow temperature fixing toners. For example, attempts to produce tonersutilizing polyester resins as binder are disclosed in U.S. Pat. No.3,590,000 to Palermiti et al. and U.S. Pat. No. 3,681,106 to Burns etal. The minimum fixing temperature of polyester binder resins can berendered lower than that of other materials, such as styrene-acrylicresins. However, this may lead to a lowering of the hot offsettemperature and, as a result, decreased offset resistance. In addition,the glass transition temperature of the resin may be decreased, whichmay cause the undesirable phenomenon of blocking of the toner duringstorage.

To prevent fuser roll offsetting and to increase fusing latitude oftoners, modification of the binder resin structure by conventionalpolymerization processes, for example, by branching, crosslinking, andthe like, has been attempted. For example, in U.S. Pat. No. 3,681,106 toBurns et al., a process is disclosed whereby a polyester resin wasimproved with respect to offset resistance by non-linearly modifying thepolymer backbone by mixing a trivalent or more polyol or polyacid withthe monomer to generate branching during polycondensation. However, anincrease in degree of branching may result in an elevation of theminimum fix temperature. Thus, any initial advantage of low temperaturefix may be diminished.

Another method of improving offset resistance is by crosslinking duringpolymerization. In U.S. Pat. No. 3,941,898 to Sadamatsu et al., forexample, a crosslinked vinyl type polymer prepared using conventionalcrosslinking was used as the binder resin. Similar disclosures for vinyltype resins are presented in U.S. Pat. No. Re. 31,072 (a reissue of U.S.Pat. No. 3,938,992) to Jadwin et al., U.S. Pat. No. 4,556,624 to Gruberet al., U.S. Pat. No. 4,604,338 to Gruber et al., and U.S. Pat. No.4,824,750 to Mahalek et al. Also, disclosures have been made ofcrosslinked polyester binder resins using conventional polycondensationprocesses for improving offset resistance, such as for example in U.S.Pat. No. 3,681,106 to Burns et al.

While significant improvements can be obtained in offset resistance andentanglement resistance, a major drawback may ensue with crosslinkedresins prepared by conventional polymerization, for example, vinyl typeprocesses including solution, bulk, suspension and emulsionpolymerizations and polycondensation processes. In all of theseprocesses, monomer and crosslinking agent are added to the reactor atthe same time. The crosslinking reaction is not very fast and chains cangrow in more than two directions at the crosslinking point by theaddition of monomers. Three types of polymer configurations areproduced--a linear and soluble portion called the linear portion, acrosslinked portion which is low in crosslinking density and thereforeis soluble in some solvents, such as, tetrahydrofuran, toluene, and thelike, and is called sol, and a portion comprising highly crosslinked gelparticles which is not substantially soluble in any solvent, forexample, tetrahydrofuran, toluene and the like, and is called gel. Thesecond portion with low crosslinking density (sol) is responsible forwidening the molecular weight distribution of the soluble part whichresults in an elevation of the minimum fixing temperature of the toner.Also, a drawback of these processes, which are not carried out underhigh shear, is that as more crosslinking agent is used the gel particlesor very highly crosslinked insoluble polymer with high molecular weightincrease in size. The large gels can be more difficult to dispersepigment in, causing unpigmented toner particles during pulverization,and toner developability may thus be hindered. Also, in the case ofvinyl polymers, the toners produced often show vinyl offset.

In U.S. Pat. No. 4,533,614 to Fukumoto et al., a process was utilizedfor preparing loosened crosslinked polyester binder resin which showedlow temperature fix and good offset resistance. Metal compounds wereused as crosslinking agents. Similar disclosures are presented in U.S.Pat. No. 3,681,106 to Burns et al. and Japanese Laid-open PatentApplications Nos. 94362/1981, 116041/1981 and 166651/1980. As discussedin the '614 patent, incorporation of metal complexes, however, caninfluence unfavorably the charging properties of the toner. Also, in thecase of color toners other than black, for example cyan, metal complexescan adversely affect the color of the pigments. It is also known thatmetal containing toner can have disposal problems in some areas, such asin the State of California, U.S.A. Metal complexes are often alsoexpensive materials.

Reactive extrusion processes for producing engineering plastics areknown, for both initial polymerization reactions employing monomers orprepolymers, and for polymer modifying reactions, such as graft,coupling and degradation reactions. However, it is believed that theprior art does not disclose the use of a reactive extrusion process toprepare crosslinked thermoplastic resins for use in toners.

In U.S. Pat. No. 4,894,308 to Mahabadi et al. and U.S. Pat. No.4,973,439 to Chang et al., for example, extrusion processes aredisclosed for preparing electrophotographic toner compositions in whichpigment and charge control additive were dispersed into the binder resinin the extruder. However, in each of these patents, there is nosuggestion of a chemical reaction occurring.

An injection molding process for producing crosslinked synthetic resinmolded articles is disclosed in U.S. Pat. No. 3,876,736 to Takiura inwhich polyolefin or polyvinyl chloride resin and crosslinking agent wasmixed in an extruder, and then introduced into an externally heatedreaction chamber outside the extruder wherein the crosslinking reactionoccurred at increased temperature and pressure, and at low or zeroshear.

In U.S. Pat. No. 4,089,917 to Takiura et al., an injection molding andcrosslinking process is disclosed in which polyethylene resin andcrosslinking agent were mixed in an extruder and reacted in reactionchambers at elevated temperature and pressure. Heating of the resinmixture occurred partially by high shear in inlet flow orifices.However, the crosslinking reaction still took place in the reactionchambers at low or zero shear, and the final product is a thermosetmolded part, and thus, is not useful for thermoplastic toner resins.

A process for dispensing premixed reactive precursor polymer mixturesthrough a die for the purposes of reaction injection molding or coatingis described in U.S. Pat. No. 4,990,293 to Macosko et al. in whichpolyurethane precursor systems were crosslinked in the die and not inthe extruder. The dimensions of the die channel were determined suchthat the value of the wall shear stress was greater than a criticalvalue in order to prevent gel buildup and consequent plugging of thedie. The final product is a thermoset molded part, and thus, is notuseful for thermoplastic toner resins.

The processes disclosed in U.S. Pat. Nos. 3,876,736, 4,089,917, and4,990,293 are not considered reactive extrusion processes, since, forexample, the crosslinking occurs in a die or a mold, and not in anextruder. These processes are for producing engineering plastics such asthermoset materials which cannot be remelted once molded, and thus arenot suitable for toner application.

SUMMARY OF THE INVENTION

Embodiments of the present invention overcome the above-discussedproblems in the prior art. The present invention provides a reactivemelt mixing process to produce low cost and safe crosslinkedthermoplastic binder resins for toner compositions which have low fixtemperatures and high offset temperatures, and which show minimized orsubstantially no vinyl offset. In this process, unsaturated base resinsor polymers are crosslinked in the molten state under high temperatureand high shear conditions, in for example an extruder, producingsubstantially uniformly dispersed microgels with controlled crosslinkingdensity, preferably using a mixture comprised of a free radicalinitiator as a crosslinking initiator, a low molecular weightunsaturated or vinyl monomer compound as a crosslinking agent or spacer,and optionally an unreactive diluent compound, hereafter referred to asthe diluent resin or wax, to moderate the initiation behavior and affectof the free radical initiator compound, and which process providescrosslinked product resin with minimized or no residual initiator ormonomer material remaining in the resin after the high temperature andhigh shear reactive extrusion crosslinking.

The present invention provides an economical, robust and reproducibleprocesses for preparing resins for toner, by batch or continuousprocesses. In embodiments of the present invention, crosslinking isaccomplished quickly to form microgel particles during melt mixing. Highshear conditions disperse resultant microgels substantially uniformly inthe polymer melt and prevent the microgels from continuing to increasein size with increasing degree of crosslinking.

In embodiments of the present invention, a reactive resin, hereinafterreferred to as base resin, for example, an unsaturated linear polyesterresin, is crosslinked in the molten state under high temperature andhigh shear conditions, preferably using a free radical initiator, forexample, an organic peroxide, as a crosslinking initiator in thepresence of a vinyl monomer crosslinking agent or spacer, in a batch orcontinuous melt mixing device, without forming any significant amountsof residual materials. Thus, the removal of byproducts or residualunreacted materials is minimized or eliminated in embodiments of thepresent invention. In other embodiments of the process, the base resinand free radical initiator, or unreactive resin or wax diluted freeradical initiator, are preblended and fed to a melt mixing device suchas an extruder at an upstream location, or the base resin and initiatorare each fed separately to the melt mixing device, for example, anextruder at either upstream or downstream locations. In an extruderscrew configuration, extruder length and temperature control may be usedto enable the initiator to be well dispersed in the polymer melt beforethe onset of crosslinking, and further, which provide a sufficient, butshort, residence time for the crosslinking reaction to be accomplished.Adequate temperature control enables the crosslinking reaction to becarried out in a highly controllable and reproducible fashion. Extruderscrew configuration and length can also provide high shear conditionsthat distribute microgels, formed during the crosslinking reaction, wellin the polymer melt, and to keep the microgels from inordinatelyincreasing in size with increasing degree of crosslinking. An optionaldevolatilization zone may be used to remove any volatiles, if desired.The resulting crosslinked polymer melt may then be pumped through a dieto a pelletizer.

The process of the present invention can be utilized to produce a lowcost, safe crosslinked toner resin with substantially no unreacted vinylmonomer or residual byproducts of crosslinking, and which can besufficiently fixed at low temperature by hot roll fixing to affordenergy saving, is particularly suitable for high speed fixing, showsexcellent offset resistance and wide fusing latitude, for example, lowfix temperature and high offset temperature, and shows minimized or novinyl offset.

Furthermore, the distance between the base resin chains can be closelycontrolled by the size of crosslinking segments or spacers which aredetermined by the choice of the type and concentration of vinyl monomeradded to the melt reaction. This enables controlled crosslink densityvariation which can influence the polymer light scattering propertiesand can be manifested for example, as gloss or matte finishes in thetoner images. Also the ratio of vinyl monomer to reactive resin can beadjusted to help control the glass transition temperature of the finalpolymer. This provides for the use of a variety of reactive base resins.Thus, a lower T_(g) base resin composition can be utilized, and with theincorporation of a higher T_(g) vinyl monomer by crosslinking, the finaltoner resin has a satisfactory blocking temperature whereas the finaltoner resin if fabricated from a physical blend of the base polymerwithout the crosslinking reaction of the present invention and ahomopolymer derived from the crosslinking vinyl monomer has tonerblocking disadvantages.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially schematic cross-sectional view of a reactiveextrusion apparatus suitable for the process of the present invention.

FIG. 2 depicts the effect of temperature on melt viscosity of varioustoner resins. Viscosity curve A is for a base resin which is a linear(non-crosslinked) unsaturated polyester resin with low fix temperatureand very low fusing latitude and is not suitable for hot roll fusing.Viscosity curves B and C are for crosslinked polyester resins preparedby the process of the present invention which resins possess low fixtemperatures and good fusing latitudes. The resin of curve C has ahigher gel content than that of curve B.

FIG. 3 depicts the effect of crosslinking on the melt viscosity ofresins prepared by a conventional crosslinking approach. Viscosity curveA is for a linear (non-crosslinked) unsaturated polyester resin with lowfix temperature and very low fusing latitude. Viscosity curve B is foran unsaturated polyester resin crosslinked by conventional methods whichdoes not employ interchain spacer units, which resin has a good fusinglatitude, but also has a high fixing temperature.

FIG. 4 shows the effect of crosslinking on the melt viscosity of tonersbased on crosslinked polyester resins of the present invention. Curve Ais a control and shows the melt viscosity of an un-crosslinked polyestertoner resin formulated with carbon black. Curve B shows the same resinformulated with carbon black as in A, which resin was crosslinked with amixture of styrene and butyl acrylate and 1.2 weight percent benzoylperoxide according to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention provides a process for fabricating low fixtemperature toner resins by reactive melt mixing in any melt mixingdevice, batch or continuous, but preferably continuous such as, forexample, an extruder wherein polymer base resins are crosslinked at hightemperature and under high shear conditions, preferably using a mixturecomprised of a free radical initiator compound, an optional unreactiveresin or wax diluent, and a vinyl monomer compound as a crosslinkingagent. Crosslinked toner resins prepared by a process related to thepresent invention are disclosed in detail in the aforementioned U.S.Pat. No. 5,227,460 (D/91117Q), the disclosure of which is hereby totallyincorporated herein by reference. Use of diluted free radical initiatorsis disclosed in Copending Application U.S. Ser. No. 08/131,250 (D/92556)filed Oct. 4, 1993, entitled "Melt Mixing Processes" the disclosure ofwhich is hereby totally incorporated herein by reference.

Low fix temperature toner resins are fabricated in embodiments, of thepresent invention, by a reactive melt mixing process comprising: (a)melting a reactive base resin, thereby forming a polymer melt; (b)adding to said polymer melt a mixture comprised of a free radicalinitiator compound or free radical initiator compound diluted with anunreactive second resin, and a vinyl monomer compound to form a secondmelt; and (c) heating and mixing under high shear said second melt toform a crosslinked and partially gelled toner resin.

In a preferred embodiment, the process comprises: forming a mixture bypreblending the reactive base resin, and a high temperature half lifefree radical initiator compound; feeding the mixture to an extruder;gently heating the mixture at a temperature of from about 70° to about120° C. to melt the base resin and to further disperse the free radicalinitiator compound therein but without significant free radicaldegradation or initiation; heating to a temperature of from about 130°to about 200° C. to cause free radical initiator degradation withreaction of the mixture while simultaneously introducing to the meltmixture a vinyl monomer; keeping the resulting polymer melt mixture inthe extruder for a sufficient residence time, for example, 10 seconds toabout 10 minutes, at a given temperature such that the required amountof interchain spacer type crosslinking is achieved; providingsufficiently high shear during the crosslinking reaction thereby keepingthe gel particles formed during crosslinking small in size and welldistributed in the polymer melt; optionally devolatilizing the melt toremove any effluent volatiles; and pumping the resulting crosslinkedresin melt through a die to a pelletizer and thereafter forming tonerparticles.

In the process of the present invention, the fabrication of thecrosslinked resin may be carried out in a melt mixing device such as anextruder described in U.S. Pat. No. 4,894,308 to Mahabadi et al., thedisclosure of which is hereby totally incorporated herein by reference.Generally, any high shear, high temperature melt mixing device suitablefor processing polymer melts may be employed, provided that theobjectives of the present invention are achieved. Examples of continuousmelt mixing devices include single screw extruders or twin screwextruders, continuous internal mixers, gear extruders, disc extrudersand roll mill extruders. Examples of batch internal melt mixing devicesinclude Banbury mixers, Brabender mixers, and Haake mixers.

One suitable type of extruder is a fully intermeshing corotating twinscrew extruder such as, for example, the ZSK-30 twin screw extruder,available from Werner & Pfleiderer Corporation, Ramsey, N.J., U.S.A.,which has a screw diameter of 30.7 millimeters and a length-to-diameter(L/D) ratio of 37.2. The extruder can melt the base resin, mix the freeradical initiator compound or resin diluted free radical initiator aloneor alternatively with a vinyl monomer crosslinking agent present intothe base resin melt, provide high temperatures in the range of 140° to200° C. and adequate residence time, for example, 10 seconds to about 10minutes, for the crosslinking reaction to be carried out, control thereaction temperature via appropriate temperature controls along theextruder channel, optionally devolatilize the melt to remove anyeffluent volatiles if needed, and pump the crosslinked polymer meltproduct through a die such as a strand die to a pelletizer. For chemicalreactions in highly viscous materials, reactive extrusion isparticularly efficient, and is advantageous because it requires nosolvents, and thus is easily environmentally controlled. It is alsoadvantageous because it permits a high degree of initial mixing of baseresin and initiator to take place, and provides an environment wherein acontrolled high temperature, which is adjustable along the length of theextruder, is available so that a very quick reaction can occur. It alsoenables a reaction to take place continuously, and thus the reaction isnot limited by the disadvantages of a batch process, wherein thereaction must be repeatedly stopped so that the reaction products may beremoved and the apparatus cleaned and prepared for a subsequentreaction. As soon as the desired amount of crosslinking is achieved, thereaction products can be quickly and continuously removed from thereaction chamber.

For a better understanding of the present invention, a typical reactiveextrusion apparatus suitable for the process of the present invention isillustrated in FIG. 1. FIG. 1 shows a twin screw extrusion device 1containing a drive motor 2, a gear reducer 3, a drive belt 4, anextruder barrel 5, a screw 6, a screw channel 7, an upstream supply portor hopper 8, a downstream supply port 9, a downstream devolatilizer 10,a heater 11, a thermocouple 12, a die or head pressure generator 13, anda pelletizer 14. The barrel 5 consists of modular barrel sections, eachseparately heated with heater 11 and temperature controlled bythermocouple 12. With modular barrel sections, it is possible to locatefeed ports and devolatilizing ports at required locations, and toprovide segregated temperature control along the screw channel 7. Thescrew 6 is also modular, enabling the screw to be configured withmodular screw elements and kneading elements having the appropriatelengths, pitch angles, and the like, in such a way as to provide optimumconveying, mixing, reaction, devolatilizing and pumping conditions.

In operation, the components to be reacted and extruded, for example,the base resin and free radical initiator or mixture of either freeradical initiator compound or resin diluted free radical initiatorcompound and vinyl monomer, enter the extrusion apparatus from the firstupstream supply port 8 and/or second downstream supply port 9. The baseresin, usually in the form of solid pellets, chips, granules, or otherforms can be fed to the first upstream supply port 8 and seconddownstream supply port 9 by starve feeding, gravity feeding, volumetricfeeding, loss-in-weight feeding, or other known feeding methods. Feedingof the free radical initiator to the extruder depends in part on thenature of the free radical initiator. In one embodiment of theinvention, especially if the free-radical initiator compound is a solid,the base resin and initiator compound are preblended prior to beingadded to the extruder, and the preblend, the base resin and/oradditional initiator may be added through either upstream supply port 8,downstream supply port 9, or both. In another embodiment, especially ifthe initiator is a liquid, the reactive base resin and free radicalinitiator can preferably be added to the extruder separately throughupstream supply port 8, downstream supply port 9, or both. This does notpreclude other methods of adding the base resin and initiator to theextruder. After the base resin and free-radical initiator have been fedinto screw channel 7, the resin is melted and the initiator is dispersedinto the molten resin as it is heated, but preferably still at atemperature below what is needed for crosslinking processes toefficiently occur. Heating takes place from two sources: (1) externalbarrel heating from heaters 11, and (2) internal heating from viscousdissipation within the polymer melt itself. When the temperature of themolten resin, initiator, and vinyl monomer reach a critical point, onsetof the crosslinking reaction takes place. It is preferable, although notabsolutely necessary, that the time required for completion of thecrosslinking reaction not exceed the residence time in the screw channel7. The rotational speed of the extruder screw preferably ranges fromabout 50 to about 500 revolutions per minute. If needed, volatiles maybe removed through downstream devolatilizer port 10 by, for example,applying a vacuum. At the end of screw channel 7, the crosslinked resinis pumped in molten form through die 13, such as for example a stranddie, to pelletizer 14 such as, for example, a water bath pelletizer,underwater granulator, etc.

With further reference to FIG. 1, the rotational speed of the screw 6can be of any suitable value provided that the objectives of the presentinvention are achieved. Generally, the rotational speed of screw 6 isfrom about 50 revolutions per minute to about 500 revolutions perminute. The barrel temperature, which is controlled by thermocouples 12and generated in part by heaters 11, is from about 40° C. to about 250°C. The temperature range for mixing the base resin and free radicalinitiator and optionally vinyl monomer in the upstream barrel zones isfrom about the melting temperature of the base resin to below thecrosslinking onset temperature, and preferably within about 40° C. ofthe melting temperature of the base resin. For example, for anunsaturated polyester base resin the temperature is preferably about 90°C. to about 130° C. The temperature range for the crosslinking reactionin the downstream barrel zones is above the crosslinking onsettemperature and the base resin melting temperature, preferably withinabout 150° C. of the base resin melting temperature. For example, for anunsaturated polyester base resin, the temperature is preferably about70° C. to about 250° C. The die or head pressure generator 13 generatespressure from about 50 pounds per square inch to about 500 pounds persquare inch. In one embodiment, the screw is allowed to rotate at about100 revolutions per minute, the temperature along barrel 5 is maintainedat about 70° C. in the first barrel section and 160° C. furtherdownstream, and the die pressure is about 50 pounds per square inch.

When crosslinking in a batch internal melt mixing device, the residencetime is preferably in the range of about 10 seconds to about 5 minutes.The rotational speed of a rotor in the device is preferably about 10 toabout 500 revolutions per minute.

Thus, in a process of the present invention, a reactive base resin inadmixture with a free radical initiator compound or resin diluted freeradical initiator compound, and a vinyl monomer compound, are fed to areactive melt mixing apparatus and vinyl monomer based crosslinking iscarried out at high temperature and high shear to produce a crosslinkedresin which enables the preparation of low fix temperature toners withgood fusing latitude and vinyl offset properties.

The base resin used in the process of this invention is, for example, areactive polymer, preferably a linear reactive polymer such as, forexample, linear unsaturated polyester. In preferred embodiments, thebase resin has a degree of unsaturation of about 0.1 to about 65 molepercent, preferably about 1 to about 50 mole percent. In a preferredembodiment, the linear unsaturated polyester base resin is characterizedby number-average molecular weight (M_(n)) as measured by gel permeationchromatography (GPC) in the range typically from 1,000 to about 20,000,and preferably from about 2,000 to about 5,000, weight average molecularweight (M_(w)) in the range typically from 2,000 to about 40,000, andpreferably from about 4,000 to about 15,000. The molecular weightdistribution (M_(w) /M_(n)) is in the range typically from about 1.5 toabout 6, and preferably from about 2 to about 4. Onset glass transitiontemperature (T_(g)) as measured by differential scanning calorimetry(DSC) is in the range typically from 50° C. to about 70° C., andpreferably from about 51° C. to about 60° C. Melt viscosity as measuredwith a mechanical viscometer at 10 radians per second is from about5,000 to about 200,000 poise, and preferably from about 10,000 to about100,000 poise at 100° C., and drops sharply with increasing temperatureto from about 100 to about 5,000 poise, and preferably from about 250 toabout 2,000 poise, as temperature rises from 100° C. to 130° C.

Linear unsaturated polyesters used as the base resin are in embodimentsof the present invention low molecular weight condensation polymerswhich may be formed by the step-wise reactions between both saturatedand unsaturated diacids, diesters or anhydrides and dihydric alcoholssuch as glycols or diols. The resulting unsaturated polyesters arereactive, that is crosslinkable, in two respects: (i) unsaturation sites(double bonds) along the polyester backbone chain; and (ii) functionalgroups such as carboxyl, hydroxy, and the like, groups amenable toacid-base or condensation reactions. Typical unsaturated polyestersuseful for this invention are prepared by melt polycondensation or otherpolymerization processes using diacids, diesters and/or anhydrides anddiols. Suitable diacids and anhydrides include but are not limited tosaturated diacids and/or anhydrides such as, for example, succinic acid,glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid,sebacic acid, isophthalic acid, terephthalic acid,hexachloroendomethylene tetrahydrophthalic acid, phthalic anhydride,chlorendic anhydride, tetrahydrophthalic anhydride, hexahydrophthalicanhydride, endomethylene tetrahydrophthalic anhydride,tetrachlorophthalic anhydride, tetrabromophthalic anhydride, and thelike, diesters derived therefrom and mixtures thereof; and unsaturateddiacids and/or anhydrides such as, for example, maleic acid, fumaricacid, chloromaleic acid, itaconic acid, citraconic acid, mesaconic acid,maleic anhydride, diesters thereof, and the like, and mixtures thereof.Suitable diols include, but are not limited to, for example, propyleneglycol, ethylene glycol, diethylene glycol, neopentyl glycol,dipropylene glycol, dibromoneopentyl glycol, propoxylated bisphenol-A,ethoxylated bisphenol-A, 2,2,4-trimethylpentane-1,3-diol, tetrabromobisphenol dipropoxy ether, 1,4-butanediol, 1,3 butanediol, and the like,and mixtures thereof, soluble in highly dissolving solvents such as, forexample, tetrahydrofuran, toluene, and the like.

Preferred linear unsaturated polyester base resins are prepared fromdiacids, diesters and/or anhydrides such as, for example maleicanhydride, fumaric acid, and the like and mixtures thereof, and diolssuch as, for example, propoxylated bisphenol-A, propylene glycol,1,3butanediol, and the like, and mixtures thereof. A particularly preferredpolyester is poly(propoxylated bisphenol A fumarate).

Substantially any suitable unsaturated polyester can be used in theprocess of the invention, including unsaturated polyesters known for usein toner resins and including unsaturated polyesters whose propertiespreviously made them undesirable or unsuitable for use as toner resinsbut which adverse properties are eliminated or reduced by crosslinkingthe unsaturated polyesters by processes of the present invention.

Any appropriate initiation technique for crosslinking can be used in theprocess of the invention. Free radical initiators such as, for example,organic peroxides or azo compounds are preferred for this process.Suitable organic peroxides include diacyl peroxides such as, forexample, decanoyl peroxide, lauroyl peroxide and benzoyl peroxide,ketone peroxides such as, for example, cyclohexanone peroxide and methylethyl ketone, alkyl peroxyesters such as, for example, t-butyl peroxyneodecanoate, 2,5-dimethyl 2,5-di(2-ethyl hexanoyl peroxy)hexane, t-amylperoxy 2-ethyl hexanoate, t-butyl peroxy 2-ethyl hexanoate, t-butylperoxy acetate, t-amyl peroxy acetate, t-butyl peroxy benzoate, t-amylperoxy benzoate, o,o-t-butyl o-isopropyl monoperoxy carbonate,2,5-dimethyl 2,5di(benzoyl peroxy)hexane, o,o-t-butyl o-(2-ethylhexyl)monoperoxy carbonate, and o,o-t-amyl o-(2-ethyl hexyl)monoperoxycarbonate, alkyl peroxides such as, for example, dicumyl peroxide,2,5-dimethyl 2,5-di(t-butylperoxy)hexane, t-butyl cumyl peroxide,α-α-bis(t-butyl peroxy)diisopropyl benzene, di-t-butyl peroxide and2,5-dimethyl 2,5-di(t-butyl peroxy)hexyne-3, alkyl hydroperoxides suchas, for example, 2,5-dihydro peroxy 2,5-dimethyl hexane, cumenehydroperoxide, t-butyl hydroperoxide and t-amyl hydroperoxide, and alkylperoxyketals such as, for example, n-butyl 4,4-di(t-butylperoxy)valerate, 1,1-di(t-butyl peroxy)3,3,5-trimethyl cyclohexane,1,1-di(t-butyl peroxy)cyclohexane, 1,1-di(t-amyl peroxy)cyclohexane,2,2-di (t-butyl peroxy) butane, ethyl 3,3-di (t-butyl peroxy)butyrateand ethyl 3,3-di(t-amyl peroxy)butyrate. Suitable azo compounds include2,2'-azobis(isobutyronitrile), 2,2'-azobis(2,4-dimethyl valeronitrile),2,2'-azobis(methyl butyronitrile), 1,1'-azobis(cyano cyclohexane) andother similar known compounds.

The resin diluted free radical initiator compound can be any of theabove radical initiators diluted in unreactive matrix resin. Forexample, useful diluent resins include these resins or waxes withoutolefinic double bonds, carboxylic acids or hydroxyl groups, otherfunctional groups which are not readily abstracted in free radicalreactions under the conditions of the present process, and are selectedfrom the group consisting of methyl terpolymer, a polyvinylidinefluoride, a polymethyl methacrylate, hydrogenatedpoly(styrene-butadiene), polyisobutylmethacrylate, polyacrylate,polymethacrylate, polystyrene, polystyrene acrylate, polystyrenemethacrylate, polyvinyl chloride, a wax component with a weight averagemolecular weight from about 1,000 to about 6,000, and mixtures thereof.

The vinyl monomer compounds useful in the present invention can be anyvinyl monomer or mixture of monomers that are readily polymerizable byfree radical species thereby functioning as a crosslinking "spacer"agent, and which monomers are selected from the group consisting ofstyrene and derivatives thereof, conjugated dienes and derivativesthereof, acrylates and derivatives thereof, and mixtures thereof.

In embodiments of the present invention, the fraction of the freeradical initiator compound to the base resin can be from about 0.01 toabout 10 percent. The weight fraction of a mixture of free radicalinitiator compound and vinyl monomer to the base resin can be from about2 to about 50 percent. The weight fraction of vinyl monomer to the baseresin can be from about 1.0 to about 40 percent. The weight fraction ofthe optional diluent resin or wax to the free radical initiator can befrom about 20 to about 300 percent. The vinyl monomer to free radicalinitiator mole ratio can be from about 1:1 to about 10,000:1.

Preferably, the rate of reaction of free radical species generated onthe backbone of the polymer chain by the free radical initiator, withthe vinyl monomer crosslinking agent, is more rapid than a competingdirect interchain coupling reaction. The following discussion ofreactivity ratios illustrates the importance of the relative ratio ofreaction. It is known that the reactivity ratios for vinylpolymerization favors the heteromonomer reaction of styrene to fumarateand fumarate to styrene rather than the homomonomer reaction of fumarateto fumarate, as disclosed in "UNSATURATED POLYESTERS:STRUCTURE ANDPROPERTIES", by Herman V. Boenig, p. 17, Elsevier Publishing Co., NewYork, (1964). The letters r₁ and r₂, respectively, represent relativemonomer reactivity ratios defined by the equations: ##EQU1## wherein agrowing chain, ending in m₁, the addition of M₁ represents k₁₁, whereina growing chain, ending in m₁, the addition of M₂ represents k₁₂,wherein a growing chain, ending in m₂, the addition of M₂ representsk₂₂, and wherein a growing chain, ending in m₂, the addition of M₁represents k₂₁. The reactivity therefore of adding either styreneradicals to fumarate double bonds, or fumarate radicals to styrenedouble bonds is a highly favored and a predominant reaction pathway andoccurs in preference to the formation of homopolymers of fumarate orhomopolymers of styrene. Although not wanting to be limited by theory,it is believed that the foregoing discussion of reactivity ratios isapplicable to preferred monomer reactions and products formed thereby inembodiments of the present invention.

In the crosslinking reaction which occurs in processes of the presentinvention at high temperature and high shear, the free radicalinitiator, such as for example benzoyl peroxide, disassociates to formfree radicals which preferentially attack the linear unsaturated baseresin polymer chains, for example, at double bonds, to form polymericradicals or radical sites on the polymer chain backbone. Crosslinkingoccurs as these polymeric radical sites react with vinyl monomer ormonomers in close proximity thereto to a limited extent in preference toother unsaturated chains or other polymeric radicals, and thereafter theresulting chain pendant styryl radicals or oligomeric styryl radicalsreact preferentially intermolecularly with unsaturated bonds orequivalent available radical chain species in the reactive base resinforming high molecular weight gel particles with controlled crosslinkingdensity. In the crosslinking reaction which occurs in the process of thepresent invention at high temperature and high shear, and in thepresence of monomers, the chemical initiator, for example benzoylperoxide, disassociates to form free radicals (step I) which attack thelinear unsaturated base resin polymer chains, for example, at doublebonds to form polymeric radicals (step II) which in turn readily reactswith one or more molecules of vinyl monomer, for example, styrene toform monomer or oligomeric grafted vinyl monomer chain radical species.Crosslinking of the present invention is believed to occur as theseoligomeric graft radicals further react with other unsaturated chains orother monomer or polymeric radicals (step III), forming high molecularweight but loosely crosslinked gel particles, that is the polymericchains are not in close proximity to one another since one or more vinylmonomers provide "spacer" units which lowers the crosslink densitycompared to a directly crosslinked sample prepared in the absence of thevinyl monomer oligomer crosslink spacer. ##STR1##

The crosslinking which occurs in the process of the invention is terizedby at least one reactive site, that is one unsaturation or double bond,within a polymer chain reacting substantially directly, with one or morevinyl monomer(s), to form a grafted monomer or oligomeric units. Thefinal step in the crosslinking reaction sequence may occur by a numberof mechanisms. Without intending to be limited by theory, it is believedthat the crosslinking process of the present invention occurspredominantly through the sequence shown in the aforementioned steps Ithrough III. The ##STR2## single headed arrow or "fishhook" representsfree radical species attack on an olefin or another free radicalresulting in covalent bond formation. Alternatively, the free radicalspecies can abstract an atom, but this is a less preferred pathway inview of the aforementioned reactivity ratios.

For example, an exemplary propoxylated bisphenol A fumarate unsaturatedpolymer undergoes the first step (step I) in the crosslinking sequencewith a free radical initiator, for example, benzoyl peroxide obtained ina pelletized form, thereby producing a free radical on the polymer resinchain backbone which in turn preferentially attacks a vinyl monomer inthe immediate vicinity and in turn produces monomeric or oligomericvinyl graft radical species. As the vinyl monomer is consumed theoligomeric graft species begin to preferentially react intermolecularlywith other unsaturated sites on another base polymer resin by furtherradical addition reactions or by hydrogen abstraction reactions as shownin the accompanying scheme below, and which chain abstraction productcan further react with other free radical species and contribute totermination of the crosslinking process and additional intermolecularcrosslinking. ##STR3##

This manner of crosslinking between chains will produce a high molecularweight intermolecularly bonded network of polyester resin molecules thatare connected yet separated by vinyl monomer spacer linkages, ultimatelyforming an open gel, wherein m₁ (or m₂ not shown ), and n represent thenumber of segments of the polymer resin that participate in the hydrogenabstraction/termination process. In preferred embodiments of theexemplary polyester, m₁ and m₂ are at least 1 and the sum of m₁ and m₂is not greater than 3. Specifically, m₁ and m₂ are independently from 1to 3, and n is from about 8 to 11.

A small concentration of free radical initiator is adequate inembodiments to accomplish the intermolecular spacing and crosslinkingprocess of the present invention, usually in the range of from about0.01 to about 10, and preferably about 10 percent by weight of initiatorin the crosslinkable reactive base resin, and preferably in the range offrom about 0.1 to about 4 percent by weight of initiator in thecrosslinkable resin. By effecting the crosslinking in the melt state athigh temperature and high shear in a melt mixing device, such as anextruder, the gel particles formed during crosslinking are kept small,that is submicron, for example, less than about 0.2 micron primaryparticle diameters as determined by light scattering, and the size ofthe particle does not grow or very minimal growth is achieved withincreasing degree of crosslinking. Also, the high shear enables themicrogel particles to be substantially uniformly dispersed in thepolymer melt.

An advantage of using a free radical initiator as the crosslinkinginitiator is that by utilizing low concentrations of initiator, forexample, less than about 10 percent by weight and in embodimentspreferably less than about 4 percent by weight, and carrying out thecrosslinking at high temperature, little or no unreacted initiatorremains in the product, and therefore, the residual contaminantsproduced in the crosslinking reaction are minimized.

Thus, the crosslinked resin produced in processes of the presentinvention is a clean and safe polymer mixture comprising reacted vinylmonomer crosslinked gel particles and a non-crosslinked or linearportion but substantially no sol. The gel content of the crosslinkedresin ranges from about 0.001 to about 50 percent by weight, andpreferably from about 0.1 to about 40 percent by weight, and morepreferably from about 3 to about 10 percent by weight for colored tonersand from about 20 to about 40 percent by weight for black toners,wherein the gel content is defined as follows: ##EQU2##

There is substantially no crosslinked polymer which is not gel, that is,low crosslink density polymer or sol, as would be obtained inconventional crosslinking processes such as, for example,polycondensation, bulk, solution, suspension, emulsion and suspensionpolymerization processes.

The crosslinked portions of the crosslinked resin product consistessentially of very high molecular weight microgel particles with highdensity crosslinking and interpolymeric chain separation from vinylmonomer grafting and crosslinking reactions as measured by gel contentand which particles are not soluble in substantially any solvents suchas, for example, tetrahydrofuran, toluene, and the like. The microgelparticles are highly crosslinked polymers with a short crosslinkinterpolymeric separation distance of about one to ten vinyl monomers.

The linear portions of the crosslinked resin have substantially the samenumber average molecular weight (M_(n)), weight average molecular weight(M_(w)), molecular weight distribution (M_(w) /M_(n)), onset glasstransition temperature (T_(g)) and melt viscosity as the base resin.Thus in embodiments, the entire crosslinked resin product has an onsetglass transition temperature of from about 50° C. to about 70° C., andpreferably from about 51° C. to about 60° C., and a melt viscosity offrom about 5,000 to about 200,000 poise, and preferably from about20,000 to about 100,000 poise, at 100° C. and from about 10 to about20,000 poise at 160° C.

In the preferred embodiment of a crosslinked unsaturated polyester resinprepared by processes of this invention, the crosslinked resin enablesthe preparation of toners with minimum fix temperatures in the range ofabout 100° C. to about 200° C., preferably about 100° C. to about 160°C., more preferably about 110° to about 150° C. Also, these low fixtemperature toners have fusing latitudes ranging from about 10° C. toabout 120° C. and preferably more than about 20° C., and morepreferably, more than about 30° C. Processes of the invention canproduce toner resins and thus toners with minimized or substantially novinyl offset.

Crosslinked polymers so produced have the important rheological propertyof allowing a toner prepared therefrom to show low fix temperature andhigh offset temperature. The low fix temperature is a function of themolecular weight and the molecular weight distribution of the linearportion, and is believed not to be significantly affected by the amountof microgel or degree of crosslinking in the resin. This is demonstratedby the close proximity of both viscosity curves A and B at lowtemperature, for example at about 80° C. as shown in FIG. 4 forun-crosslinked and crosslinked unsaturated polyester. The hot offsettemperature is increased by the presence of microgel particles whichimpart elasticity to the resin. Thus, with a higher degree ofcrosslinking or gel content, the hot offset temperature increases. Thisis reflected in divergence of the viscosity curves at highertemperature, for example, at 120° C. as also shown in FIG. 4. As thedegree of crosslinking or gel content increases, the low temperaturemelt viscosity does not change significantly while the high temperaturemelt viscosity increases considerably. In an exemplary embodiment, thehot offset temperature can increase approximately 30% relative to theunmodified resin. This is achieved by crosslinking in the melt state athigh temperature and high shear, for example, in an extruder resultingin the formation of microgel alone, distributed substantially uniformlythroughout the linear portion, and no intermediates which arecrosslinked polymers with low crosslinking density (sol). Whencrosslinked intermediate polymers are generated by conventionalpolymerization processes, the viscosity curves shift to the right inparallel from a low to high degree of crosslinking as shown in FIG. 3.This is reflected in increased hot offset temperature, but may alsoindicate an increase in minimum fix temperature.

In addition to providing a unique rheological property to the tonerresin not attainable by conventional crosslinking processes forpreparing toner resins, the reactive melt mixing processes of thepresent invention have several other important advantages. By choosingthe type and molecular weight properties of the base resin, the minimumfix temperature can be easily manipulated. The hot offset temperaturecan also be easily manipulated by controlling the gel content in thecrosslinked resin which can be regulated by the amount of free radicalinitiator, and vinyl monomer fed to the extruder and/or regulating theextruder process conditions such as, for example, feed rate, screwrotational speed, barrel temperature profile, screw configuration, andlength. Thus, it is possible to produce a series of resins and thustoners with the same MFT, but with different fusing latitudes.Crosslinking by the use of free radical initiators in the extruder is avery clean means of modifying resin, since very low concentrations ofinitiators are used, often less than 4 percent by weight, and theresidual contaminants of the crosslinking reaction are minimal.

The resins are generally present in the toner in an amount of from about40 to about 98 percent by weight, and more preferably from about 70 toabout 98 percent by weight, although they may be present in greater orlesser amounts, provided that the objectives of the invention areachieved. For example, toner resin produced by processes of the presentinvention can be subsequently melt blended or otherwise mixed with acolorant, charge carrier additives, surfactants, emulsifiers, pigmentdispersants, flow additives, and the like. The resultant product canthen be pulverized by known methods such as milling to form tonerparticles. The toner particles preferably have an average volumeparticle diameter of about 5 to about 25, more preferably about 5 toabout 15 microns.

Various suitable colorants can be employed in toners of the invention,including suitable colored pigments, dyes, and mixtures thereofincluding carbon black, such as Regal 330® carbon black (Cabot),Acetylene Black, Lamp Black, Aniline Black, Chrome Yellow, Zinc Yellow,Sicofast Yellow, Luna Yellow, Novaperm Yellow, Chrome Orange, BayplastOrange, Cadmium Red, Lithol Scarlet, Hostaperm Red, Fanal Pink,Hostaperm Pink, Lithol Red, Rhodamine Lake B, Brilliant Carmine,Heliogen Blue, Hostaperm Blue, Neopan Blue, PV Fast blue, CinquassiGreen, Hostaperm Green, titanium dioxide, cobalt, nickel, iron powder,Sicopur 4068 FF, and iron oxides such as Mapico Black (Columbia), NP608and NP604 (Northern Pigment), Bayferrox 8610 (Bayer), MO8699 (Mobay),TMB-100 (Magnox), mixtures thereof and the like.

The colorant, preferably carbon black, cyan, magenta and/or yellowcolorant, is incorporated in an amount sufficient to impart the desiredcolor to the toner. In general, pigment or dye is employed in an amountranging from about 2 to about 60 percent by weight, and preferably fromabout 2 to about 7 percent by weight for color toner and about 5 toabout 60 percent by weight for black toner.

Various known suitable effective positive or negative charge enhancingadditives can be selected for incorporation into the toner compositionsproduced by the present invention, preferably in an amount of about 0.1to about 10, more preferably about 1 to about 3, percent by weight.Examples include quaternary ammonium compounds inclusive of alkylpyridinium halides; alkyl pyridinium compounds, reference U.S. Pat. No.4,298,672, the disclosure of which is totally incorporated hereby byreference; organic sulfate and sulfonate compositions, U.S. Pat. No.4,338,390, the disclosure of which is totally incorporated hereby byreference; cetyl pyridinium tetrafluoroborates; distearyl dimethylammonium methyl sulfate; aluminum salts such as Bontron E84™ or E88™(Hodogaya Chemical); and the like.

Additionally, other internal and/or external additives may be added inknown amounts for their known functions.

The resulting toner particles optionally can be formulated into adeveloper composition by mixing with carrier particles. Illustrativeexamples of carrier particles that can be selected for mixing with thetoner composition prepared in accordance with the present inventioninclude those particles that are capable of triboelectrically obtaininga charge of opposite polarity to that of the toner particles.Accordingly, in one embodiment the carrier particles may be selected soas to be of a negative polarity in order that the toner particles whichare positively charged will adhere to and surround the carrierparticles. Illustrative examples of such carrier particles includegranular zircon, granular silicon, glass, steel, nickel, iron ferrites,silicon dioxide, and the like. Additionally, there can be selected ascarrier particles nickel berry carriers as disclosed in U.S. Pat. No.3,847,604, the entire disclosure of which is hereby totally incorporatedherein by reference, comprised of nodular carrier beads of nickel,characterized by surfaces of reoccurring recesses and protrusionsthereby providing particles with a relatively large external area. Othercarriers are disclosed in U.S. Pat. Nos. 4,937,166 and 4,935,326, thedisclosures of which are hereby totally incorporated hereby byreference.

The selected carrier particles can be used with or without a coating,the coating generally being comprised of fluoropolymers, such aspolyvinylidene fluoride resins, terpolymers of styrene, methylmethacrylate, a silane, such as triethoxy silane, tetrafluoroethylenes,other known coatings and the like.

The diameter of the carrier particles is generally from about 50 micronsto about 1,000 microns, preferably about 200 microns, thus allowingthese particles to possess sufficient density and inertia to avoidadherence to the electrostatic images during the development process.The carrier particles can be mixed with the toner particles in varioussuitable combinations. Best results are obtained when about 1 part tonerto about 10 parts to about 200 parts by weight of carrier are mixed.

Toners produced by the process of the invention can be used in knownelectrostatographic imaging methods, although the fusing energyrequirements of some of those methods can be reduced in view of theadvantageous fusing properties of the subject toners as discussedherein. Thus, for example the toners or developers can be charged, e.g.,triboelectrically, and applied to an oppositely charged latent image onan imaging member such as a photoreceptor or ionographic receiver. Theresultant toner image can then be transferred, either directly or via anintermediate transport member, to a support such as paper or atransparency sheet. The toner image can then be fused to the support byapplication of heat and/or pressure, for example with a heated fuserroll at a temperature lower than 200° C., preferably lower than 150° C.

The invention will further be illustrated in the following, nonlimitingexamples, it being understood that these examples are intended to beillustrative only and that the invention is not intended to be limitedto the materials, conditions, process parameters and the like recitedherein. Parts and percentages are by weight unless otherwise indicated.

EXAMPLE I

A crosslinked unsaturated polyester resin is prepared by the reactiveextrusion process by melt mixing 94.3 parts of a linear unsaturatedpolyester with the following structure: ##STR4## wherein n is the numberof repeating units and having M_(n) of about 4,000, M_(w) of about10,300, M_(w) /M_(n) of about 2.58 as measured by GPC, onset T_(g) ofabout 55° C. as measured by DSC, and melt viscosity of about 29,000poise at 100° C. and about 750 poise at 130° C. as measured at 10radians per second, 5.0 parts styrene monomer and 0.7 parts benzoylperoxide initiator as outlined in the following procedure.

The unsaturated polyester resin and benzoyl peroxide initiator areblended in a rotary tumble blender for 30 minutes. The resulting drymixture is then fed into a Werner & Pfleiderer ZSK-30 twin screwextruder, with a screw diameter of 30.7 mm and a length-to-diameter(L/D) ratio of 37.2, at 10 pounds per hour using a loss-in-weightfeeder. The styrene monomer is continuously added via the downstreamaddition port in an equivalent ratio relative to the fed mixture ofresin and peroxide, that is 95:5 parts of feed to styrene monomer. Thecrosslinking is carried out in the extruder using the following processconditions: barrel temperature profile of 70/140/140/140/140/140/140°C., die head temperature of 140° C., screw speed of 100 revolutions perminute and average residence time of about three minutes. The extrudatemelt, upon exiting from the strand die, is cooled in a water bath andpelletized. The product which is a crosslinked polyester has an expectedonset T_(g) of about 54° C. as measured by DSC, melt viscosity of about40,000 poise at 100° C. and about 400 poise at 160° C. as measured at 10radians per second, a gel content of about 7.0 weight percent and aprimary microgel particle size of about 0.1 micron as determined lightscattering.

The linear and crosslinked portions of the product are separated bydissolving the product in tetrahydrofuran and filtering off themicrogel. The dissolved part is reclaimed by evaporating thetetrahydrofuran. This linear part of the resin, when characterized byGPC, is expected to have M_(n) of about 3,900, M_(w) of about 10,100,M_(w) /M_(n) of about 2.59, and onset T_(g) of about 54° C. which issubstantially the same as the original non-crosslinked resin, whichindicates that it contains no sol.

Thereafter, a toner is formulated by melt mixing the above preparedcrosslinked unsaturated polyester resin, 92 percent by weight, with 6percent by weight carbon black and 2 percent by weight alkyl pyridiniumhalide charge enhancing additive in a Haake batch mixer. The toner ispulverized and classified to form a toner with an average particlediameter of about 9.1 microns and a geometric size distribution (GSD) ofabout 1.32. The toner is evaluated for fixing, blocking, and vinyloffset performance. The cold offset temperature is about 110° C., theminimum fix temperature is about 126° C., the hot offset temperature isabout 135° C., and the fusing latitude is about 9° C. Also, the tonerhas excellent blocking performance, with a T_(g) of about 53° C. asmeasured by DSC, and shows no apparent vinyl offset.

EXAMPLE II

A crosslinked unsaturated polyester resin is prepared by the reactiveextrusion process by melt mixing 91.8 parts of a linear unsaturatedpolyester with the structure and properties described in Example I, 5.0parts styrene monomer and 2.8 parts diluent resinpolyalpha-methylstyrene (2,000 molecular weight T_(g) =51° C.), and 1.4parts benzoyl peroxide initiator as outlined in the following procedure.

The unsaturated polyester resin and preblended resin diluted benzoylperoxide initiator are blended in a rotary tumble blender for 30minutes. The resulting dry mixture is then fed into a Werner &Pfleiderer ZSK-30 twin screw extruder at 10 pounds per hour using aloss-in-weight feeder. The crosslinking is carried out in the extruderusing the following process conditions: barrel temperature profile of70/160/160/160/160/160/160° C., die head temperature of 160° C., screwrotational speed of 100 revolutions per minute and average residencetime of about three minutes. The styrene is added as described inExample I. The extrudate melt, upon exiting from the strand die, iscooled in a water bath and pelletized. The product which is acrosslinked polyester has an expected onset T_(g) of about 54° C. asmeasured by DSC, melt viscosity of about 65,000 poise at 100° C. andabout 12,000 poise at 160° C. as measured at 10 radians per second, agel content of about 50 weight percent, and a primary microgel particlesize of about 0.1 micron as determined by light scattering.

The linear and crosslinked portions of the product are separated bydissolving the product in tetrahydrofuran and filtering off themicrogel. The dissolved part is reclaimed by evaporating thetetrahydrofuran. This linear part of the resin, when characterized byGPC, is found to have M_(n) of about 3,900, M_(w) of about 10,100, M_(w)/M_(n) of about 2.59, and onset T_(g) of 54° C. which is substantiallythe same as the original non-crosslinked resin, which indicates that itcontains no sol.

Thereafter, a toner is prepared and evaluated according to the sameprocedure as in Example I except that the average particle diameter isabout 9.8 microns and the GSD is about 1.33. The cold offset temperatureis about 110° C., the minimum fix temperature is about 135° C., the hotoffset temperature is about 195° C., and the fusing latitude is about60° C. Also, the toner has excellent blocking performance (T_(g) about53° C. as measured by DSC) and shows no apparent vinyl offset.

Comparative Example I

This comparative example shows the effect of changes in gel content ontoner fixing performance for crosslinked unsaturated polyester resins.Two resins are compared in this example. Resin A is linear unsaturatedpolyester with the structure and properties of the linear unsaturatedpolyester used as the base resin as described in Example I. Resin B isstyrene crosslinked polyester resin prepared by the reactive extrusionprocess by melt mixing 94.0 parts linear unsaturated polyester (ResinA), 5.0 parts styrene and 1.0 part benzoyl peroxide initiator asoutlined in the following procedure.

The unsaturated polyester resin (Resin A), styrene, and benzoyl peroxideinitiator are blended in a rotary tumble blender for 30 minutes. Theresulting dry mixture is then fed into a Werner & Pfleiderer ZSK-30 twinscrew extruder at 10 pounds per hour using a loss-in-weight feeder. Thecrosslinking is carried out in the extruder using the following processconditions: barrel temperature profile of 70/160/160/160/160/160/160°C., die head temperature of 160° C., screw rotational speed of 100revolutions per minute and average residence time of about threeminutes. The extrudate melt, upon exiting from the strand die, is cooledin a water bath and pelletized.

Thereafter, Toners A and B are prepared from the resins A and B,respectively, and evaluated according to the same procedure as inExample I. The toner of resin A has an average particle diameter ofabout 9.3 microns and a GSD of about 1.29. The toner of resin B has anaverage particle diameter of about 10.1 microns and a GSD of about 1.32.Results of fixing tests are shown in Table 1. Results for Toner Aproduced from Resin A show a cold offset temperature of about 110° C.and a hot offset temperature of about 120° C. Due to the proximity ofCOT and HOT, it is not possible to accurately determine the minimum fixtemperature, indicating that the fusing latitude is very small. FromTable 2, it is seen that with a vinyl monomer crosslinked toner resin ofthe present invention, the fusing latitude is elevated considerably,while the minimum fix temperature remains virtually unchanged.

                                      TABLE 1                                     __________________________________________________________________________    Linear     Sol   Gel                                                          Content    Content                                                                             Content                                                                            COT                                                                              MFT  HOT                                                                              FL                                           Wt. %      Wt. % Wt. %                                                                              °C.                                                                       °C.                                                                         °C.                                                                       °C.                                   __________________________________________________________________________    Toner A                                                                             100  0      0   110                                                                              --   120                                                                              --                                           Toner B                                                                              85  0     15   110                                                                              129  155                                                                              26                                           __________________________________________________________________________

Comparative Example II

This comparative example shows the difference between crosslinkedpolyester resins prepared by a conventional crosslinking without vinylmonomer method versus the vinyl monomer crosslinked resin preparedaccording to the present invention. Two additional resins (C and D shownin FIG. 3 as curves A and B, respectively) are considered in thisexample, a linear polyester and a crosslinked polyester prepared byconventional crosslinking.

First, a linear polyester resin, Resin C, is prepared by the followingprocedure. About 1,645 grams of dimethyl terephthalate, 483 grams of1,2-propane diol, and 572 grams of 1,3-butane diol are charged to athree liter, four necked resin kettle which is fitted with athermometer, a stainless steel stirrer, a glass inlet tube and a fluxcondenser. The flask is supported in an electric heating mantle. Argongas is allowed to flow through the glass inlet tube thereby sparging thereaction mixture and providing an inert atmosphere in the reactionvessel. The stirrer and heating mantle are activated and the reactionmixture is heated to about 80° C. at which time about 0.96 grams oftetraisopropyl titanate is added to the reaction mixture. The reactionmixture is gradually heated to a temperature of about 170° C. whereuponmethanol from the condensation reaction is condensed and is removed asit is formed. As the reaction progresses and more methanol is removed,the reaction temperature is slowly increased to about 200° C. Over thisperiod, about 94 weight percent of the theoretical methanol is removed.At this time, the reactor is cooled to room temperature and the reactoris modified by replacing the reflux condenser with a dry ice-acetonecooled trap with the outlet of the trap connected to a laboratory vacuumpump through an appropriate vacuum system. Heat is reapplied to thereactor with the reactants under argon purge. As the reactants becomemolten, stirring is started. When the reactants are heated to about 84°C. the vacuum is about 30 microns mercury. The reaction is continued atabout these conditions for about seven hours until the mixture become soviscous that considerable difficulty is encountered in removing theentrapped volatile reaction by-products from the mixture. At this point,the vacuum is terminated by an argon purge and the reaction product iscooled to room temperature. The resulting polymer is found to have ahydroxyl number of about 48, an acid number of about 0.7, a methyl esternumber of about 7.5 and a glass transition temperature of about 56° C.Using vapor pressure osmometry in methyl ethyl ketone, the numberaverage molecular weight of the resulting linear polymer is found to beabout 4,100.

Second, a crosslinked polyester resin, Resin D, is prepared bypolyesterification by the following procedure. About 1,645 grams ofdimethyl terephthalate, 483 grams of 1,2-propane diol, 572 grams of1,3-butane diol and 15 grams of pentaerythritol as condensationpolymerization branching agent are charged to a three liter, four neckedresin kettle and the polyesterification and branching are carried outunder the same conditions as above. The resulting polymer is found tohave a hydroxyl number of about 48, an acid number of about 0.7, amethyl ester number of about 7.5 and a glass transition temperature ofabout 56° C. By dissolution in chloroform and filtration through a 0.22micron MF Millipore filter under air pressure, the polymer is found tocontain about 16 weight percent gel. Using vapor pressure osmometry inmethyl ethyl ketone, the number average molecular weight of the solublefraction of the polymer is found to be about 6,100 which is comprised oflinear polymer with a number average molecular weight of about 4,200 andsol.

Thereafter, Toners C and D are prepared from the two resins, C and D,respectively, and evaluated according to the same procedure as inExample I. Results of fixing tests are shown in Table 2 along with theresults for a toner of Resin B. The toner particles of Resin C have anaverage particle diameter of about 8.7 microns and a GSD of about 1.30,while those of Resin D have an average particle diameter of about 10.5microns and a GSD of about 1.31. The hot offset temperature increases(32° C.) with increasing degree of branching (sol and gel content is30%). However, this is also accompanied by an increase in minimum fixtemperature resulting in only a small increase in fusing latitude (10°C.). Most of the benefit achieved by crosslinking is lost due to theincrease in minimum fix temperature. Also in Table 3 are the results offusing evaluations for Toner B, a styrene monomer crosslinkedunsaturated polyester resin of the present invention (see ComparativeExample I for details). With Toner B, the fusing latitude increasesdramatically with increasing gel content and without increasing solcontent, while the minimum fix temperature remains virtually unchanged.

                                      TABLE 2                                     __________________________________________________________________________    Linear     Sol   Gel                                                          Content    Content                                                                             Content                                                                            COT                                                                              MFT  HOT                                                                              FL                                           Wt. %      Wt. % Wt. %                                                                              °C.                                                                       °C.                                                                         °C.                                                                       °C.                                   __________________________________________________________________________    Toner B                                                                             85   0     15   110                                                                              129  155                                                                              26                                           Toner C                                                                             100  0      0   110                                                                              --   120                                                                              --                                           Toner D                                                                             70   14    16   120                                                                              146  156                                                                              10                                           __________________________________________________________________________

EXAMPLE III Polyester Base Resin Preparation

An unsaturated polyester base resin was prepared by a conventionalpolycondensation reaction which was carried out in a three liter, fournecked resin kettle fitted with a thermometer, a stainless steelstirrer, a glass inlet tube and a reflux condenser. The flask issupported and heated in an electric mantle. The ingredients were addedwith a liquid diol introduced first, followed by a solid ester and ananhydride. The ingredients were 20 moles of 1,3 butanediol, 9.37 molesof dimethyl terephthalate, and 0.625 moles of maleic anhydride. It isknown that the maleate easily isomerizes to the fumarate form under theconditions typical for a polycondensation reaction so the product ofthis reaction is referred to as a fumarate. The mixture was heatedslowly until most of the ingredients were melted and then slow stirringwas begun. The temperature was held at 145° C. with an argon sparge for30 minutes. At that time 0.016 moles of isopropyl titanate catalyst wasadded. The argon was continuously bubbled through the stirred mixtureand the stirring speed was increased while methanol and water werecollected. The temperature was raised slowly to about 200° C. withstirring until about 95% of the theoretical methanol and water werecollected. The mixture was cooled, stirring was stopped and theglassware was changed to glassware suitable for application of a vacuum.The mixture was again heated, and stirring slowly increased while avacuum was applied. The 1,3-butanediol was removed and collected in coldtraps as the condensation polymerization continued and the polymer chainlengths increased in size. The reaction was carried out at 200° C. withaverage pressure of about 100 microns of vacuum. Samples of polymer wereperiodically withdrawn to determine the extent of the reaction bymeasurement of the melt flow index of the polymer product. The abovereaction was continued until a 1,3-butyleneterephthalate cofumaratecompound with a T_(g) of 38° C. and Melt Flow Index of 15 at 105° C.using 2.16 Kg weight was obtained. The rheology of this linearcondensation polymer is shown by curve A in FIG. 4. A toner was madefrom this linear polyester and 10% by weight of BP1300 carbon black.This toner had a T_(g) of 45° C. and the toner blocked at ambientconditions in the container.

EXAMPLE IV

Unsaturated base polyester, 49.4 parts, as described and prepared inExample III was crosslinked with a mixture of 49.4 parts of a mixture ofstyrene (77 weight %) and butyl acrylate (23 weight %) and 1.2 weightpercent benzoyl peroxide initiator. This polymer showed two T_(g) valuesof 35 and 51. The melt rheology for this crosslinked material is shownby curve B in FIG. 4. A toner was made according to the procedure ofExample I from a mixture of this crosslinked polyester and 10% by weightof BP1300 carbon black. The toner had a T_(g) of 46 and 58 and hadimproved blocking properties. The blocking temperature for this tonerwas 115° F.

The disclosures of all the above mentioned patents and publicationsmentioned herein are incorporated by reference in their entirety.

Other modifications of the present invention may occur to those skilledin the art subsequent to a review of the present application. Theaforementioned modifications, including equivalents thereof, areintended to be included within the scope of the present invention.

While this invention has been described with reference to particularpreferred embodiments, the invention is not limited to the specificexamples given, and other embodiments and modifications can be made bythose skilled in the art without departing from the spirit and scope ofthe invention.

What is claimed is:
 1. A reactive melt mixing process for preparingtoner resin, comprising:(a) melting a reactive base resin, therebyforming a first polymer melt; (b) adding to said polymer melt a freeradical initiator compound to form a second melt; and (c) heating andmixing under high shear said second melt to form a third melt containinga partially crosslinked and partially gelled toner resin and wherein afree radical reactive vinyl monomer compound is added to either (b) or(c) as a crosslinking component.
 2. The process of claim 1, wherein saidprocess is accomplished in a batch melt mixing process.
 3. The processof claim 1, wherein said process is accomplished in a continuous meltmixing process.
 4. The process of claim 1, wherein said process isaccomplished in a reactor subsequent to formation of the base resin andwherein the base resin is formed by a condensation polymerizationreaction.
 5. The process of claim 1, wherein said reactive base resin isa linear unsaturated resin.
 6. The process of claim 5, wherein saidlinear unsaturated resin is a polyester with a number average molecularweight (M_(n)) as measured by gel permeation chromatography (GPC) in therange from 1,000 to about 20,000, a weight average molecular weight(M_(w)) in the range from 2,000 to about 40,000, a molecular weightdistribution (M_(w) /M_(n)) in the range from about 1.5 to about 6, anonset glass transition temperature (T_(g)) as measured by differentialscanning calorimetry in the range from 50° C. to about 70° C., and amelt viscosity as measured with a mechanical viscometer at 10 radiansper second from about 5,000 to about 200,000 poise at 100° C., said meltviscosity dropping with increasing temperature to about 100 to about5,000 poise at 130° C.
 7. The process of claim 5, wherein said linearunsaturated resin is a polyester prepared from (a) diacids, diesters oranhydrides selected from the group consisting of succinic acid, glutaricacid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacicacid, isophthalic acid, terephthalic acid, hexachloroendomethylenetetrahydrophthalic acid, phthalic anhydride, chlorendic anhydride,tetrahydrophthalic anhydride, hexahydrophthalic anhydride, endomethylenetetrahydrophthalic anhydride, tetrachlorophthalic anhydride,tetrabromophthalic anhydride, maleic acid, fumaric acid, chloromaleicacid, itaconic acid, citraconic acid, mesaconic acid, maleic anhydride,and ester derivatives thereof, and mixtures thereof, and (b) diolsselected from the group consisting of propylene glycol, ethylene glycol,diethylene glycol, neopentyl glycol, dipropylene glycol,dibromoneopentyl glycol, propoxylatedbisphenol-A2,2,2,4-trimethylpentane-1,3-diol, tetrabromobisphenoldipropoxy ether, 1,4-butanediol, 1,3 butanediol, and mixtures thereof.8. The process of claim 1, wherein said heating initiates crosslinkingof the reactive resin by reaction with said free radical initiatorcompound selected from the group consisting of organic peroxides,azobisalkylnitriles, peroxycarbonates, and mixtures thereof.
 9. Theprocess of claim 1, further comprising diluting the free radicalinitiator with a diluent resin or wax component which is free ofolefinic double bonds and which component is selected from the groupconsisting of a methyl terpolymer, a polyvinylidine fluoride, apolymethyl methacrylate, polystyrene, polystyrene acrylate copolymer,polystyrene methacrylate copolymer, hydrogenated poly(styrene-butadiene)copolymer, polyisobutylmethacrylate, polyacrylate, polymethacrylate,saturated polyester, polyvinyl chloride, and a wax component with aweight average molecular weight from about 1,000 to about 6,000, andmixtures thereof.
 10. The process of claim 1, wherein said vinyl monomercompound is readily polymerized onto the base resin by free radicalspecies located on the base resin thereby functioning as a grafting andsubsequently a crosslinking agent, and is selected from the groupconsisting of styrene and derivatives thereof, conjugated dienes andderivatives thereof, acrylates and derivatives thereof, and mixturesthereof.
 11. The process of claim 1, wherein the weight fraction of saidfree radical initiator compound to said base resin is from about 0.01 toabout 10 percent.
 12. The process of claim 9, wherein the weightfraction of said mixture of free radical initiator compound and optionalvinyl monomer compound, with or without a diluent resin or wax, to saidbase resin is from about 2 to about 50 percent.
 13. The process of claim1, further comprising mixing said reactive base resin and said freeradical initiator compound and optionally said vinyl monomer prior toforming said polymer melt.
 14. The process of claim 1, furthercomprising mixing said free radical initiator compound and optionallysaid vinyl monomer into said polymer melt at a temperature less thanabout the onset of crosslinking temperature (T_(c)), thereby producinguniform dispersion of the initiator in said polymer melt prior tocrosslinking of said polymer melt.
 15. The process of claim 14, furthercomprising initiating crosslinking of said polymer melt with saidmixture of free radical initiator and optionally said vinyl monomer byraising the temperature of said polymer melt above about the onset ofcrosslinking temperature (T_(c)), and maintaining the temperature in therange of about 130° to about 200° C. of said polymer melt during saidcrosslinking.
 16. The process of claim 14, further comprising the stepof initiating crosslinking of said polymer melt with said mixture offree radical initiator compound and optionally said vinyl monomer byraising the temperature of said polymer melt above about the onset ofcrosslinking temperature (T_(c)) and within 150° C. of the base resinmelting temperature, and maintaining the temperature in the range ofabout 130° to about 200° C. of said polymer melt during saidcrosslinking.
 17. The process of claim 1, wherein said heating leadingto free radical addition polymerization type crosslinking is effected tocompletion.
 18. The process of claim 1, wherein said heating and mixingis accomplished in an extruder.
 19. The process of claim 5, wherein saidlinear unsaturated resin is a polyester of a propoxylated bisphenol Afumarate.
 20. The process of claim 9, further comprising preblendingsaid reactive base resin and a free radical initiator compoundoptionally in admixture with a vinyl monomer and an unreactive diluentresin or wax to form a preblend, and feeding said preblend, andoptionally additional base resin and optionally additional free radicalinitiator compound to a continuous melt mixing apparatus.
 21. Theprocess of claim 20, further comprising preblending said reactive baseresin and a free radical initiator compound, optionally in admixturewith a vinyl monomer and an unreactive diluent resin or wax, to form apreblend, and feeding said preblend, and optionally additional freeradical initiator compound initiator to a batch internal melt mixingapparatus.
 22. The process of claim 1, wherein said resulting tonerresin is a polyester resin comprising crosslinked portions and linearportions, wherein said crosslinked portions comprise high molecularweight microgel particles with a gel content of about 0.1 to about 40weight percent, wherein said gel particles are less than about 0.2micron in primary particle size diameter and are substantially uniformlydistributed in said resin, and wherein said linear portions are linearunsaturated polyesters having a number average molecular weight (M_(n))as measured by gel permeation chromatography in a range of from about1,000 to about 20,000, a weight average molecular weight (M_(w)) of fromabout 2,000 to about 40,000, a molecular weight distribution (M_(w)/M_(n)) of about 1.5 to about 6, a glass transition temperature (T_(g))as measured by differential scanning calorimetry in the range of fromabout 50° C. to about 70° C., and a melt viscosity as measured with amechanical viscometer at 10 radians per second from about 5,000 to about200,000 poise at 100° C., said melt viscosity dropping with increasingtemperature to about 100 to about 5,000 poise at 130° C.
 23. The processof claim 1, wherein said resulting toner resin is a polyester resincomprising vinyl monomer crosslinked portions and linear polyesterportions, wherein said crosslinked portions are in the form of microgelsless than about 0.2 micron in primary particle size diameter and aresubstantially uniformly distributed in said resin, wherein the amount ofcrosslinked portions or gel content is in the range from about 0.001 toabout 50 percent by weight of said toner resin, wherein the amount ofreactive linear resin is in the range of about 50 to about 99.999percent by weight of said toner resin, and wherein said toner resin hasa glass transition temperature in the range from about 50° C. to about70° C., and melt viscosity at 10 radians per second from about 5,000 toabout 200,000 poise at 100° C. and from about 10 to about 20,000 poiseat 160° C.
 24. The process of claim 1, wherein said toner resin providesa minimum fix temperature of a toner comprised of said resin and pigmentfrom about 100° C. to about 160° C., a hot offset temperature of fromabout 110° C. to about 220° C., and which toner has substantially novinyl offset.
 25. The process of claim 1, further comprising the step offorming solid toner particles from said crosslinked toner resin.
 26. Theprocess of claim 25, further comprising the step of combining carrierparticles with said toner particles to form a developer.
 27. The processof claim 1, wherein said toner resin is combined with at least onemember selected from the group consisting of a colorant, a chargecontrol additive, a surfactant, an emulsifier, and a pigment dispersantto form a mixture, and said mixture is further melt blended to form atoner.
 28. The process of claim 27, wherein said colorant is selectedfrom the group consisting of cyan, magenta, yellow, red, green, blue,carbon black, and magnetite, and mixtures thereof.
 29. The process ofclaim 27, wherein said charge control additive is selected from thegroup consisting of alkyl pyridinium halides, distearyl dimethylammonium methyl sulfate, and metallic alkyl salicylates, and mixturesthereof.
 30. The process of claim 1, wherein substantially all of saidcrosslinking is accomplished under high shear mixing and heating. 31.The process of claim 1, wherein free radical initiator compound has ahalf life of about one hour at a temperature from about 80° C. to about170° C.
 32. A reactive melt mixing process for preparing a low fixtemperature toner resin, comprising: preblending a reactive base resin,a high temperature half life free radical initiator compound andoptionally an unreactive resin or wax as a free radical initiatordiluent; feeding the mixture to an extruder; heating the mixture at atemperature of from about 70° C. to about 120° C. to melt the base resinand to further disperse the free radical initiator therein but withoutsignificant free radical initiator degradation or; simultaneously addingto the melt mixture a vinyl monomer for forming crosslinked segmentscomprised of said vinyl monomers between adjacent reactive resinmolecules; heating to a higher temperature of from about 130° C. toabout 200° C. to cause free radical initiator degradation; retaining thepolymer melt in the extruder for a sufficient residence time of fromabout 10 seconds to about 10 minutes to complete the crosslinking;providing sufficiently high shear during the vinyl monomer interchaincrosslinking reaction thereby keeping the gel particles formed duringcrosslinking small in size and well distributed in the polymer melt;optionally devolatilizing the melt to remove any effluent volatiles; andpumping the crosslinked resin melt through a die to a pelletizer.
 33. Aprocess in accordance with claim 1 wherein the vinyl monomer to freeradical initiator mole ratio is from about 1:1 to about 10,000:1.
 34. Aprocess in accordance with claim 1 wherein the weight fraction of vinylmonomer to the base resin is from about 1.0 to about 40 percent.
 35. Aprocess in accordance with claim 9 wherein the weight fraction of thediluent resin or wax component to the free radical initiator compound isfrom about 20 to about 300 percent.